The spontaneous fluctuations of the cytosolic CaィイD12+ィエD1 concentrations ([Ca]ィイD2inィエD2) have often been observed in isolated single rat adrenal chromaffin cells. To see whether these [Ca]ィイD2inィエD2 changes occur also in vivo, we measured [Ca]ィイD2inィエD2 in individual chromaffin cells involved in adrenal slice preparations after loading with Oregon-green-BAPTA-1/AM. We often observed the spontaneous changes in [Ca]ィイD2inィエD2 when measured with cofocal laser-scanning microscope. In addition, we found that the spontaneous [Ca]ィイD2inィエD2 increase immediately spreads to the neighboring cells in some cases. The synchronization of [Ca]ィイD2inィエD2 increases among the neighboring cells occurs more frequently in mouse adrenal medullary slice preparations, where the gap-junction has been shown to exist. Treatment of mouse adrenal slices with the ATP receptor antagonist PPADS did not inhibit the spread of CaィイD12+ィエD1 signals, and ATP increased [Ca]ィイD2inィエD2 in only 30% of isolated cells tested, suggesting that ATP released from the cells is not responsible for the excitation of the neighboring cells. We also investigated the effects of the inhibitors of gap-junctions on the spread of CaィイD12+ィエD1 signals, but these agents markedly inhibited the occurrence of the spontaneous [Ca]ィイD2inィエD2 increase. However, the combined loading of Lucifer yellow and Rhodamine green dextran-70000 into the cells through the damaged membrane revealed that the distribution pattern of the former agent, which passes through the gap-junctions, was broader than that of the latter. These results suggest that the synchronized [Ca]ィイD2inィエD2 changes among cells were mediated by the gap junctions between the cells.